Proefschrift_Holstein

Striatal dopamine and motivated cognitive control

Imagine for a minute that you are a squirrel and that you and your friend are in the forest, looking for nuts and berries. While collecting food, you follow a path, taking you along nut trees and berry bushes. When you encounter a fork in the road, you decide to go left and your furry squirrel friend takes the path to the right. Along the chosen path you initially encounter many nuts which you gather and hide with diligence. After a while, nut trees become increasingly scarce. Luckily, the amount of berry bushes increase, so you shift your focus to indulging in berries. Your squirrel friend on the other hand has chosen a more challenging road where nut trees and berry bushes quickly alternate. As he proceeds along his road, he will need to sometimes eat a berry, then collect nut or two, followed again by a berry. After a while your paths meet and you find yourself waiting for your friend for quite some time. Why was your fellow squirrel so much slower? Well, each time he had to switch his focus from nuts to berries and vice versa, he had to construct a new set of appropriate responses. Switching your focus (whether it is between collecting berries and nuts or between updating your Facebook status and writing your thesis) is more costly compared with repeating the same behaviour. Therefore alternating between tasks takes more time to complete. This process is known as task switching . Finally your friend arrives, you continue foraging together. Suddenly you stumble upon another Y-junction. This time your friend insists on taking the path to the left and you go right. Unfortunately, nut trees and berry bushes randomly alternate along both paths and you both need to exert quite some control over your behaviour to switch between the two tasks (i.e. finding nuts and berries). Your friend finally catches a break: the trees and bushes on his road produce enormous nuts and berries. When you reach the end and the two roads meet up again, your friend is waiting for you with a smug grim on his squirrel face. Why was your squirrel friend faster this time around? Well, your friend anticipated a higher payout for his efforts, which may have made it easier for him to alternate between tasks. Why is this and how does this work in the brain? We refer to the internal and external factors that can orient and invigorate behaviour in order to obtain a goal as reward motivation (e.g. when you can obtain large berries). We know that reward motivation can alter cognitive control , a set of processes and mental abilities allowing the pursuit of goals in a volatile and distracting environment. For example, knowing that you can obtain large berries (reward motivation) can alter the ability to quickly alternate between tasks (i.e. task switching). Moreover, the neurotransmitter dopamine plays an important role in rewardmotivation and cognitive control. Recent work has shown that dopamine also plays a role in the interaction between rewardmotivation and cognition (such as when the anticipated size of the berries and nuts alters the ability of the squirrel to quickly alternate between tasks). In the following section I will provide an overview of the status of the literature prior to the start of the experiments presented in this thesis. In addition, I will propose a hypothesized neural mechanism by which information about rewards may influence cognitive processes. In chapter 2 I will present an outline and general introduction of the work in this thesis.

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